Let me explain: say you are using 2 different ad groups with, in ad group 1, the broad keyword “sell my car” and in ad group 2 the keyword “selling my car”.

Sometimes google will trigger the ad in adgroup 1 for the keyword “selling my car”  resulting in: a lower ctr as your ad in ad group 1 won’t be as targeted for this keyword as the one in ad group 2 might be, and also can cause the following thing to happen for your keyword in ad group 2: “This particular search term is already associated with an ad from one of your other campaigns or ad groups. Only one ad per advertiser may appear on a given page per search term. When two or more of your ads are eligible for a single search term, we show the one with the highest ranking at the moment.”

There is a little trick to make sure that ads aren’t shown for keywords within another adgroup: you simply have to add keyword 2, in this case “selling my car” as a negative in ad group 1.

That way ads are only shown for the keyword within that adgroup, not for keywords in other adgroups! You then have a more targeted ad group and a better chance at improving your ctr.

To summarise you should list all of the keywords in each of your ad groups and add them as negative keywords in all of the other groups.

Let me know how this works for you and if you need more explanation, just ask your questions in the comment box below.